Optical waveguide and method for producing same

ABSTRACT

An optical waveguide having a optical waveguide path capable of securing a high light propagation characteristic regardless of the type of a supporting base, provided with a multilayer circuit board, an optical waveguide path arranged on the multilayer circuit board, a light receiving element, IC chips, and a light emitting element, the optical waveguide path formed on a transparent substrate excellent in flatness and transferred to the multilayer circuit board. The light propagation loss becomes small, and a signal to be transmitted at a high speed being transmitted as a light signal and a signal which can be transmitted at a relatively low speed being transmitted as an electrical signal, whereby the signal propagation delay which becomes the problem when a signal is transmitted by only electrical wiring is overcome, and the influence of electromagnetic noise becomes small.

RELATED APPLICATION DATA

The present application claims priority to Japanese Application No. P10-306090, filed Oct. 27, 1998, and Japanese Application No. P11-120631,filed Apr. 27, 1999, and is a divisional of U.S. application Ser. No.10/301,008 filed Nov. 21, 2002 now U.S. Pat. No. 7,163,598, whichapplication is a divisional of U.S. application Ser. No. 09/422,255,filed Oct. 21, 1999 now abandoned, all of which are incorporated hereinby reference to the extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical waveguide provided with anoptical waveguide path through the inside of which a light signal ispropagated and a method for producing the same, more particularlyrelates to a optical waveguide suitable for transmitting a light signalin an ultra high speed signal processing circuit or parallel typedigital signal processing circuit or other signal processing circuit,optical communication, and optical connection of an optical link oroptical fiber channel etc. to an optical module and a method forproducing the same.

2. Description of the Related Art

In recent years, due to the dramatic improvements in wirelesstelecommunications technology used for cellular telephones and the like,wired telecommunications technology used in integrated service digitalnetworks (ISDN) and the like, and the processing capabilities ofpersonal computers (PC) and other data processing devices and to thedigitalization of audiovisual (AV) apparatuses and so on, there has beenan ongoing movement toward transferring all kinds of media via datacommunications networks. Further, use of the Internet, first andforemost, and local area networks (LAN) and wide area networks (WAN) andother such data communications networks is now spreading in business andamong individuals. Due to this, an environment will be realized in thefuture in which household electrical appliances and AV equipment areconnected in the home through a PC to form a network in whichinformation will be able to be freely transferred through a telephoneline, CATV (cable television or community antenna television) line,ground wave television channel, or satellite broadcast or satellitecommunication channel or other data transmitting means.

In order to freely transfer image data handled at a high speed ofseveral Mbps to 10 or more Mbps by such a data transmitting means, adata transmission rate of for example about 10 Mbps to 1 Gbps isdemanded. Optical communications and transmission technology can realizesuch data transmission rate. For example, in a so-called trunk systemtelecommunications network extending over 10 km to 100 km such as anoptical cable laid on the bottom of the sea, wide use is being made ofoptical communications and transmission technology due to their low lossand economy.

On the other hand, as the means for transmitting data over a relativelyshort distance, for example, between boards in an apparatus or betweenchips on a board, use has mainly been made of wired communications andtransmitting means such as twisted pair cables or coaxial cables.Recently, although optical fiber channels and optical data links andother technology for using optical transmission have begun spreading inuse, at the present time they have not yet spread to an extent of theoptical communications and transmitting means replacing wiredcommunications and transmitting means. One of the reasons for this isthe issue of cost versus effect. Specifically, for example, there can bementioned the points that, in order to maintain the performance ofoptical communications (for example, transmission rate and transmissionquality), technology is required for precise positioning between a lightemitting element and a light receiving element constituting the opticalcommunications device and the optical fiber, that countermeasures forlight leakage, consideration of electromagnetic interference, andcountermeasures to noise are necessary, and that, as a result, thedevice becomes complex and expensive etc.

On the other hand, advances in the technology for integrated circuits(IC) and large scale integrated circuits (LSI) have led to improvementsin operating rates and scales of integration. For example, rapidimprovements are being made in the performance of microprocessors andthe capacity of memory chips. Further, amounts of data being handled byPCs connected by networks are rapidly growing as well. Accordingly, theissues have arisen of how to deal with the rise in the frequency of thesignal processing clocks, the rise in the degree of parallel operation,and the rise in the speed of the access to the memories. In view of thissituation, semiconductor chips are being miniaturized and the gatelengths of the transistors reduced along with this and the drivingcapabilities being improved so as to increase the operating rate insidesemiconductor chips. In memory access circuits and processors ofmulti-microprocessor (MPU) configurations, however, the parasiticcapacitance component of the portion which becomes necessary at the timeof mounting, for example, the package of the semiconductor device, islarge, so high speed signal transmission operation becomes difficult inthe electric wiring connected to the outside of the semiconductor chips.Further, application of a high speed signal to the electric wiring cancause spikes in the current or the voltage and can cause electromagneticinterference (EMI) and other types of electromagnetic interferencenoise, reflection noise, and crosstalk noise.

Therefore, in order to transmit high speed signals even betweensemiconductor chips on a circuit board and other short distance signaltransmission routes, for example, it is considered desirable that thesignals be transmitted by light and that, in particular, use be made ofan optical transmission and communications system using an opticalwaveguide path as the transmission path. When transmitting a signal bylight, the signal delay due to the “CR” time constant (C: capacitance ofthe wiring, R: resistance of the wiring) of the wiring can be eliminatedand the influence of electromagnetic noise can be avoided, thereforetransfer of high speed signals becomes possible. Accordingly, it isnecessary to maintain the communications performance of opticalcommunications and transmission equal to the communications performanceof wired communications and transmission, and realize a reduction of thecosts in order to promote the use of short distance opticalcommunications and transmission systems even in the field of equipmentfor general users.

In order to maintain the communications performance of opticalcommunications and transmission equal to the communications performanceof wired communications and transmission, it is required that forexample the light propagation loss of the optical waveguide path be keptsmall. As a material having a small light propagation loss satisfyingthis condition, there is a quartz-based material. Quartz has anextremely good light transmission property as has already been proved byoptical fibers. When an optical waveguide path is prepared by quartz, areduction of the loss to less than 0.1 dB/cm is achieved.

FIG. 32 is a view of an example of the configuration of an opticalwaveguide of the related art (see Japanese Unexamined Patent Publication(Kokai) No. 62-204208). This optical waveguide is comprised of a flatsilicon substrate 501 on which are formed thin film multilayer wiring505 with the wiring insulated from each other by an insulating layer 506and on which are provided an optical waveguide path 502 made of quartzand an LSI 504. Further, above each end region of the optical waveguidepath 502, a light receiving element 503 and a light emitting element(not illustrated) are formed and electrically connected to the LSI 504arranged in their vicinity. In this optical waveguide, the light signalemitted from the not illustrated light emitting element is propagatedinside the optical waveguide path 502, reflected at an end surface 502a, and made to strike the light receiving element 503.

As the method for producing an optical waveguide having such aconfiguration, there is known the method of forming the opticalwaveguide path 502 on the silicon substrate 501 on which the thin filmmultilayer wiring 505 are formed, then using for example reactive ionetching (RIE) or other anisotropic etching to process the end surface502 a of the optical waveguide path 502 so as to become approximately45° with respect to the surface of the silicon substrate 501 and thenfurther mounting the light receiving element 503, light emittingelement, and LSI 504 on the silicon substrate 501.

Summarizing the disadvantage, in the method for producing the opticalwaveguide mentioned above, however, since the optical waveguide path 502made of quartz is supposed to be formed on the silicon substrate 501,thin film technology had to be used for forming the optical waveguidepath 502. Formation of the optical waveguide path 502 using thin filmtechnology results in an excellent dimensional accuracy, but converselyhas the disadvantage that the formation and processing of a film havinga thickness of no more than several μm are difficult.

Further, even if a signal can be transmitted at a high speed by light,the electric power has to continue to be supplied and various controlsignals of relatively low speed etc. have to continue to be transmittedby electrical wiring (electrical signals). For this reason, it suffersfrom the disadvantage that while it is essential to form the thin filmmultilayer wiring 505 as the electrical wiring on the silicon substrate501, this electrical wiring forming region would be too costly and poorin practicality if being the usual circuit board size (several 10s of cmsquare) or module size (several cm square).

In order to overcome the disadvantage, it can be considered to form theoptical waveguide path on a printed circuit board on which the electriccomponents can be mounted. The surface of a circuit board fabricated bysuch a thick film process, however, has thick films of metal formed byfor example a plating process and therefore has a large unevenness. Forthis reason, if the optical waveguide path is formed on such a printedcircuit board, the disadvantage arises that the uneven shape of thesurface of the board will end up having an effect on the shape of theoptical waveguide path and end up leading to an increase of the lightpropagation loss of the optical waveguide path and a reduction of thedimensional accuracy.

Further, in the wet etching or washing etc. when forming the opticalwaveguide path on a circuit board, a step of immersing the entire boardin acidic and alkaline solutions or organic solvents or the like becomesnecessary, therefore there is the problem of the board being liable tobe damaged. Further, the board is also liable to be damaged at the timeof dry etching or the time of the high temperature heat treatment.Accordingly, it is difficult to use a circuit board formed by the thickfilm process as the substrate, so it was necessary to use an expensivesubstrate having a high heat resistance and other characteristics.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical waveguidehaving an optical waveguide path capable of securing high lightpropagation characteristics regardless of the type of the supportingbase and a method for producing the same.

Another object of the present invention is to provide an opticalwaveguide in which the optical waveguide path is inexpensively formedand by which the manufacturing cost can be lowered and a method forproducing the same.

Still another object of the present invention is to provide an opticalwaveguide enabling the transmission of a further higher speed signalwhich had been difficult to realize by only electrical wiring, andcapable of improving the electromagnetic noise resistance property ofthe transmission signal and a method for producing the same.

According to a first aspect of the present invention, there is providedan optical waveguide comprising a substrate and an optical waveguidepath which is separately formed in advance so that the light signal canbe propagated inside it and then is arranged on and secured to thesubstrate.

According to a second aspect of the present invention based on thefirst, electrical wiring is formed on the substrate.

According to a third aspect of the present invention based on the first,the optical waveguide further comprises on the substrate at least one ofa light emitting element for converting an electrical signal to a lightsignal and a light receiving element for converting a light signal to anelectrical signal.

According to a fourth aspect of the present invention based on thethird, the optical waveguide further comprises on the substrate anintegrated circuit for transferring an electrical signal with the atleast one of the light emitting element and light receiving element.

According to a fifth aspect of the present invention based on the first,the optical waveguide path is formed on another substrate different fromthe substrate and then transferred from that other substrate to thesubstrate.

According to a sixth aspect of the present invention based on the first,the optical waveguide path is provided at least at one end with a lightreflecting portion having at least one of a function of reflecting alight signal from the outside into the optical waveguide path and afunction of reflecting a light signal propagated through the opticalwaveguide path out of the optical waveguide path.

According to a seventh aspect of the present invention based on thesixth, the optical waveguide path is comprised of a core layer and acladding layer and the light reflecting portion is provided at least atone end of the core layer.

According to an eight aspect of the present invention based on thesecond, the optical waveguide further comprises on the substrate atleast one of a light emitting element for converting an electricalsignal to a light signal and a light receiving element for converting alight signal to an electrical signal and an integrated circuit fortransferring an electrical signal with the at least one of the lightemitting element and light receiving element and the optical waveguidepath is formed at least at one end with a light reflecting portion forinputting or outputting a light signal.

According to a ninth aspect of the present invention based on theeighth, the electrical wiring of the substrate supplies electric powerto the at least one of a light emitting element and light receivingelement and the integrated circuit.

According to a 10th aspect of the present invention based on the eighth,the electrical wiring of the substrate is for electrically connectingthe at least one of a light emitting element and light receiving elementand the integrated circuit.

According to an 11th aspect of the present invention based on theeighth, the substrate is a multilayer circuit board comprised of aplurality of electrical wiring layers stacked via insulators.

According to a 12th aspect of the present invention based on the eighth,the optical waveguide path is comprised of a first cladding layer formedon the substrate, a core layer stacked on the first cladding layer, anda second cladding layer stacked on the core layer.

According to a 13th aspect of the present invention based on the 12th,the first cladding layer also serves as the bonding layer.

According to a 14th aspect of the present invention based on the eighth,the bonding layer is comprised of a photo-curing resin.

According to a 15th aspect of the present invention based on the eighth,the bonding layer is comprised of a heat-curing resin.

According to a 16th aspect of the present invention based on the eighth,the at least one of a light emitting element and light receiving elementand the integrated circuit are connected by connection electrodes to theelectrical wiring of the substrate.

According to a 17th aspect of the present invention based on the 16th,the connection electrodes are comprised of spherical bodies made of anelectroconductive material coated with a solder.

According to an 18th aspect of the present invention based on the 16th,the connection electrodes are comprised of a solder comprised mainly oflead (Pb) and tin (Sn).

According to a 19th aspect of the present invention based on the eighth,the optical waveguide path is one which transmits a transmitted signalat a first rate and the electrical wiring of the substrate is one whichtransmits a transmitted signal at a second rate slower than the firstrate.

According to a 20th aspect of the present invention based on the eighth,at least one of the light emitting element, light receiving element, andintegrated circuit is arranged on the substrate using the opticalwaveguide path as a spacer interposed between it and the substrate.

According to a 21st aspect of the present invention based on the eighth,the light reflecting portion is comprised of an inclined surface formedat least at one end of the optical waveguide path and has at least oneof a function of reflecting a light signal from the outside into theoptical waveguide path and a function of reflecting a light signalpropagated through the optical waveguide path outside of the opticalwaveguide path.

According to a 22nd aspect of the present invention based on the 21st,the inclined surface is inclined approximately 45° with respect to adirection of light propagation in the optical waveguide path.

According to a 23rd aspect of the present invention based on the 11th,the insulator of the multilayer circuit board is comprised of aninorganic material including at least one material selected from thegroup comprising alumina (Al₂O₃), a glass ceramic, aluminum nitride(AlN), and mullite.

According to a 24th aspect of the present invention based on the 11th,the insulator of the multilayer circuit board is comprised of an organicmaterial including at least one material selected from the groupcomprising a glass epoxy resin, a polyimide, BT resin, polyphenyl ether(PPE) resin, phenol resin, and polyolefin resin.

According to a 25th aspect of the present invention, there is provided amethod for producing an optical waveguide comprising a step of formingan optical waveguide path using a first substrate as a supporting base,a step of securing the optical waveguide path supported by the firstsubstrate together with a second substrate, and a step of removing thefirst substrate so as to transfer the optical waveguide path to thesecond substrate.

According to a 26th aspect of the present invention based on the 25th,the step of securing the optical waveguide path and the second substrateuses an adhesive.

According to a 27th aspect of the present invention based on the 26th, aphoto-curing resin is used as the adhesive.

According to a 28th aspect of the present invention based on the 27th, asubstrate comprised of a light transmitting material is used as thefirst substrate and the photo-curing resin is cured by irradiating lightthrough the first substrate.

According to a 29th aspect of the present invention based on the26th,the step of forming the optical waveguide path and the step ofsecuring the optical waveguide path and the second substrate include astep of forming a core layer, a step of forming a resin layer serving asa cladding layer so as to surround the core layer, a step of using theresin layer in the uncured state as the adhesive and bonding togetherthe optical waveguide path supported by the first substrate and thesecond substrate, and a step of curing the resin layer to form thecladding layer and simultaneously completing the securing of the opticalwaveguide path to the second substrate.

According to a 30th aspect of the present invention based on the 25th,the step of forming the optical waveguide path includes a step offorming a substrate separation layer between the first substrate and theoptical waveguide path so as to enable the first substrate to beseparated from the optical waveguide path in a later step and, in thestep of transferring the optical waveguide path to the second substrate,the substrate separation layer being removed to remove the firstsubstrate.

According to a 31st aspect of the present invention based on the 30th,the substrate separation step is formed by silicon dioxide (SiO₂).

According to a 32nd aspect of the present invention based on the 25th, asubstrate comprised of a material which can be dissolved by apredetermined solution is used as the first substrate and, in the stepof transferring the optical waveguide path to the second substrate, thefirst substrate is dissolved so as to remove it.

According to a 33rd aspect of the invention based on the 32nd, thesolution is a not more than 5 vol % hydrofluoric acid (HF) solution andthe dissolvable material is a photosensitive glass.

According to a 34th aspect of the present invention based on the 25th,the step of forming the optical waveguide path on the first substrateincludes a step of forming a plurality of optical waveguide paths spacedfrom each other and the step of securing the optical waveguide path andthe second substrate includes a step of bonding the second substrate tothe optical waveguide paths through an adhesive comprised of aphoto-curing resin, a step of irradiating light to the bonding layerthrough the first substrate to selectively expose and cure only regionsof the bonding layer corresponding to the optical waveguide paths, and astep of removing the uncured photo-curing resin on the second substrate.

According to a 35th aspect of the present invention based on the 34th,before the step of bonding the second substrate to the optical waveguidepaths, there is further a step of forming a light blocking film at theregions of the first substrate other than the regions where the opticalwaveguide paths are formed and the side surfaces of the opticalwaveguide paths, the light blocking film being used as a mask forselective exposure.

According to a 36th aspect of the present invention based on the 35th,the step of forming a light blocking film includes a step of forming apeeling layer at a surface of the optical waveguide paths to which thesecond substrate is secured, a step of forming a light blocking filmover the entire exposed surface of the first substrate, opticalwaveguide paths, and peeling layer, and a step of removing the peelinglayer so as to selectively remove the light blocking layer in contactwith the peeling layer.

According to a 37th aspect of the present invention based on the 25th,the method further comprises a step of forming on the second substrateat least one of a light emitting element for converting an electricalsignal to a light signal and a light receiving element for converting alight signal to an electrical signal.

According to a 38th aspect of the present invention based on the 37th,the method further comprises a step of forming on the second substratean integrated circuit for transferring an electrical signal with the atleast one of the light emitting element and light receiving element.

According to a 39th aspect of the present invention based on the 25th,the second substrate is an electrical circuit board formed withelectrical wiring.

According to a 40th aspect of the present invention based on the 39th,the method further comprises a step of forming on the second substrateat least one of a light emitting element for converting an electricalsignal to a light signal and a light receiving element for converting alight signal to an electrical signal and a step of forming on the secondsubstrate an integrated circuit for transferring an electrical signalwith the at least one of the light emitting element and light receivingelement.

According to a 41st aspect of the present invention based on the 40th,the method further comprises a step of sealing the at least one of alight emitting element and light receiving element and the integratedcircuit by a sealing resin material.

According to a 42nd aspect of the present invention based on the 40th,the step of forming an optical waveguide path using the first substrateas a supporting base includes a step of forming a substrate separationlayer on the first substrate so as to enable the first substrate and theoptical waveguide path to be separated, a step of forming an opticalwaveguide path on the substrate separation layer, and a step of formingat inclined surface at least at one end of the optical waveguide path.

According to a 43rd aspect of the present invention based on the 42nd,the step of forming the optical waveguide path includes a step offorming a first cladding layer on the substrate separation layer, a stepof forming a core layer on the first cladding layer, and a step offorming a second cladding layer on the core layer.

According to a 44th aspect of the present invention based on the 40th,as the second substrate, use is made of a multilayer circuit boardcontaining at least one type of inorganic material selected from thegroup comprising alumina (Al₂O₃), a glass ceramic, aluminum nitride(AlN), and mullite.

According to a 45th aspect of the present invention based on the 40th,as the second substrate, use is made of a multilayer circuit boardcontaining at least one type of organic material selected from the groupcomprising a glass epoxy resin, a polyimide, BT resin, polyphenyl ether(PPE) resin, phenol resin, and polyolefin resin.

According to a 46th aspect of the present invention based on the 40th,the second substrate is comprised of a core substrate and a printedsubstrate formed on at least one surface of the core substrate andprinted with electrical wiring patterns.

According to a 47th aspect of the present invention based on the 43rd,as the material for forming the first and second cladding layers, one isuse having a refractive index of light smaller than that of the materialforming the core layer.

According to a 48th aspect of the present invention based on the 43rd,as the material forming the first and second cladding layers, one isused including as a main ingredient at least one material selected fromthe group comprising a polyimide, epoxy resin, acryl resin, polyolefinresin, and synthetic rubber.

According to a 49th aspect of the present invention based on the 42nd,as the material forming the substrate separation layer, silicon dioxide(SiO₂) is used.

According to a 50th aspect of the present invention based on the 42nd,as the material forming the substrate separation layer, an etchablemetal material is used.

According to a 51st aspect of the present invention based on the 42nd,the step of forming an inclined surface at least at one end of theoptical waveguide path includes a step of coating the optical waveguidepath with a photoresist and exposing and developing the photoresist soas to form a desired photoresist pattern, a step of inclining an edgeregion of the photoresist pattern, a step of using the photoresistpattern as a mask and anistropically etching the portion of the opticalwaveguide path exposed from the edge region of the photoresist patternto make the end of the optical waveguide path a tapered inclinedsurface, and a step of removing the photoresist pattern.

According to a 52nd aspect of the present invention based on the 42nd,the step of forming an inclined surface at least at one end of theoptical waveguide path includes a step of vapor depositing on theoptical waveguide path a metal film, a step of coating the metal filmwith a photoresist and exposing and developing the photoresist so as toform a desired photoresist pattern, a step of using the photoresistpattern as a mask and etching the metal film to form a desired pattern,a step of using the metal film as a mask and irradiating a laser beam ona predetermined region of the optical waveguide path from apredetermined angle to cut the optical waveguide path, and a step ofremoving the metal film by etching and cleaning the processed product asa whole.

According to a 53rd aspect of the present invention based on the 42nd,the step of forming an inclined surface at least at one end of theoptical waveguide path includes a step of heating a heat tool having aninclined surface at its tip and pressing the tip of the heat tool intothe optical waveguide path so as to form the inclined surface in theoptical waveguide path and a step of removing the heat tool and thenpolishing off scum produced at the melted portion of the opticalwaveguide path.

According to a 54th aspect of the present invention based on the 42nd,the step of forming an inclined surface at least at one end of theoptical waveguide path includes a step of cutting the first substrate ata predetermined angle and a step of polishing an end surface formed bythe cutting of the first substrate so as to form the end surface of theoptical waveguide path into an inclined surface.

According to a 55th aspect of the present invention based on the 40th,as the first substrate, use is made of a substrate comprised of a lighttransmitting material able to pass light and the step of securing thesecond substrate to the optical waveguide path includes a step offorming at a predetermined position on the second substrate a bondinglayer comprised of a photo-curing resin which cures by irradiation oflight, a step of bringing the bonding layer of the second substrate intoclose contact with the optical waveguide path formed on the firstsubstrate, and a step of irradiating light from the back of the firstsubstrate toward the second substrate to cure the bonding layer.

According to a 56th aspect of the present invention based on the 40th,the step of securing the second substrate to the optical waveguide pathincludes a step of forming at a predetermined position on the secondsubstrate a bonding layer comprised of a heat-curing resin, a step ofbringing the bonding layer of the second substrate into close contactwith the optical waveguide path formed on the first substrate, and astep of heating the first substrate and the second substrate as a wholeto cure the bonding layer.

According to a 57th aspect of the present invention based on the 42nd,in the step of removing the first substrate to transfer the opticalwaveguide path to the second substrate, a solvent is supplied betweenthe bonded first substrate and second substrate to remove the substrateseparation layer and then the first substrate is separated from theoptical waveguide path.

According to a 58th aspect of the present invention based on the 42nd,in the step of removing the first substrate to transfer the opticalwaveguide path to the second substrate, a solvent is supplied betweenthe bonded first substrate and second substrate to separate the boundarybetween the substrate separation layer and the optical waveguide path.

According to a 59th aspect of the present invention based on the 40th,the at least one of the light emitting element and light receivingelement and the integrated circuit have connection electrodes and in thestep of forming the at least one of the light emitting element and lightreceiving element and the step of forming the integrated circuit, the atleast one of the light emitting element and light receiving element andthe integrated circuit are mounted on the second substrate by flip chipbonding using the connection electrodes.

According to a 60th aspect of the present invention based on the 59th,inthe step of forming the at least one of the light emitting element andlight receiving element and the step of forming the integrated circuit,the optical waveguide path is used as a spacer interposed between the atleast one of the light emitting element and light receiving element andthe integrated circuit and the second substrate

According to a 61st aspect of the present invention based on the 59th,spherical portions formed on tips of fine gold wires are pressed againstelectrodes of the at least one of the light emitting element and lightreceiving element and the integrated circuit, then the sphericalportions and the fine gold wires are pulled apart to cut them andthereby form the connection electrodes.

In short, in the optical waveguide according to the present invention,therefore, an optical waveguide path formed separately in advance isarranged on the substrate and then a light signal is propagated insidethis optical waveguide path.

Further, in the method for producing the optical waveguide according tothe present invention, therefore, an optical waveguide path formed usinga first substrate as a supporting base is secured together with a secondsubstrate and then the first substrate is removed to transfer theoptical waveguide path to the second substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clearer from the following description of the preferredembodiments given with reference to the attached drawings, in which:

FIG. 1 is a sectional view of the configuration of an optical waveguideaccording to a first embodiment of the present invention;

FIG. 2 is a perspective view for explaining a step of production of theoptical waveguide according to the first embodiment of the presentinvention;

FIG. 3 is a perspective view for explaining a step of productionfollowing FIG. 2;

FIG. 4 is a perspective view for explaining a step of productionfollowing FIG. 3;

FIG. 5A is a partially cutaway perspective view for explaining a step ofproduction following FIG. 4, and FIG. 5B is a sectional view taken alonga line VB-VB of FIG. 5A;

FIG. 6 is a sectional view for explaining a step of production followingFIGS. 5A and 5B;

FIG. 7 is a sectional view for explaining a step of production followingFIG. 6;

FIG. 8A is a partially cutaway perspective view for explaining a step ofproduction following FIG. 7, and

FIG. 8B is a sectional view taken along a line VIIIB-VIIIB of FIG. 8A;

FIG. 9A is a partially cutaway perspective view for explaining a step ofproduction following FIGS. 8A and 8B, and FIG. 9B is a sectional viewtaken along a line IXB-IXB of FIG. 9A;

FIG. 10 is a graph of the relationship between the light transmissionrate of an epoxy resin used in the first embodiment of the presentinvention and the wavelength of the light;

FIG. 11A is a partially cutaway perspective view for explaining a stepof production following FIGS. 9A and 9B, and FIG. 11B is a sectionalview taken along a line XIB-XIB of FIG. 11A;

FIG. 12 is a sectional view for explaining a step of productionfollowing FIG. 6;

FIG. 13 is a partially cutaway perspective view for explaining a step ofproduction of the optical waveguide according to a second embodiment ofthe present invention;

FIG. 14 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 13;

FIG. 15 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 14;

FIG. 16 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 15;

FIG. 17 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 16;

FIG. 18 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 17;

FIG. 19 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 18;

FIG. 20 is a partially cutaway perspective view for explaining a step ofproduction following FIG. 19;

FIG. 21 is a sectional view for explaining a step of production of anoptical waveguide according to a third embodiment of the presentinvention;

FIG. 22 is a sectional view for explaining a step of productionfollowing FIG. 21;

FIG. 23 is a sectional view for explaining a step of productionfollowing FIG. 22;

FIG. 24 is a sectional view for explaining a step of production of anoptical waveguide according to a fourth embodiment of the presentinvention;

FIG. 25 is a sectional view for explaining a step of productionfollowing FIG. 24;

FIG. 26 is a sectional view for explaining a step of productionfollowing FIG. 25;

FIG. 27 is a sectional view for explaining a step of productionfollowing FIG. 26;

FIG. 28 is a sectional view for explaining a step of production of anoptical waveguide according to a fifth embodiment of the presentinvention;

FIG. 29 is a sectional view for explaining a step of productionfollowing FIG. 28;

FIG. 30 is a sectional view for explaining a step of productionfollowing FIG. 29;

FIG. 31 is a sectional view of the configuration of an optical waveguideaccording to a sixth embodiment of the present invention; and

FIG. 32 is a sectional view of an example of the configuration of anoptical waveguide of the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, preferred embodiments of the present invention will be explainedin detail by referring to the drawings.

First Embodiment

First, an explanation will be made of the configuration of an opticalwaveguide according to a first embodiment of the present invention withreference to FIG. 1.

FIG. 1 is a view showing the sectional configuration of an opticalwaveguide 1 according to the present embodiment. This optical waveguide1 is provided with a multilayer circuit board 2, an optical waveguidepath 11 bonded to the multilayer circuit board 2 via a bonding layer 6,a light receiving element 21 mounted on the multilayer circuit board 2over the optical waveguide path 11 as a spacer, IC chips 25 and 35, anda light emitting element 31. Here, the multilayer circuit board 2corresponds to a concrete example of the “substrate” and “secondsubstrate” of the present invention. Further, the IC chips 25 and 35correspond to concrete examples of the “integrated circuit” of thepresent invention.

The multilayer circuit board 2 is an electric circuit substratecomprised of a plurality of electrical wiring 3 stacked via insulators4. The electrical wiring 3 have the function of supplying electric powerto the IC chips 25 and 35, light receiving element 21, and lightemitting element 31, and the function of transferring for example lowspeed control signals with the IC chips 25 and 35, the light receivingelement 21, and the light emitting element 31 and transferring signalswith the outside. Namely, the electrical wiring 3 are mounting usepattern wiring for mounting the light receiving element 21, lightemitting element 31, and IC chips 25 and 35 on the multilayer circuitboard 2. Specifically, various power supply wiring patterns, includingground lines, for supplying electric power to the light receivingelement 21, light emitting element 31, and IC chips 25 and 35, wiringfor connecting the IC chips 25 and 35 and the light receiving element 21and the light emitting element 31, including wiring for supplyingcontrol use signals, etc. are formed.

As the multilayer circuit board 2, use is made of for example a ceramicmultilayer circuit board in which the insulator 4 is made of alumina(Al₂O₃), a low temperature sintered glass ceramic, glass ceramic,aluminum nitride (AlN), mullite, or another inorganic material. Further,use is sometimes made of a glass epoxy multilayer circuit board in whichthe insulator 4 is made of FR-4 or another glass epoxy resin, aso-called “build up” multilayer circuit board enabling high densitypattern formation on a usual glass epoxy circuit board byphotolithography using for example a photosensitive epoxy resin, aflexible multilayer circuit board in which a polyimide film or the likeis used as the insulator 4, or a multilayer circuit board using anorganic material such as a BT resin, PPE (polyphenyl ether) resin,phenol resin, or polyolefin resin (for example Teflon® made by Dupont).Other than this, it is also possible to use a printed circuit boardcomprised of a core substrate made of for example a dielectric materialon which is arranged a printed board on which electrical wiring patternsare printed at a high density.

The bonding layer 6 is interposed between the optical waveguide path 11and the multilayer circuit board 2 and is constituted by for example aglass epoxy resin or other photo-curing resin or heat curing resin,Further, the thickness thereof is for example about 10 μm. This bondinglayer 6 also serves to flatten the unevenness of the surface of themultilayer circuit board in addition to its role of bonding the opticalwaveguide path 11 and the multilayer circuit board 2.

The optical waveguide path 11 is constituted by for example a core layer54, an upper cladding layer 53, and a lower cladding layer 55 formed soas to surround the core layer 54. This optical waveguide path 11 hasinclined surfaces 11 a and 11 b giving sharp, for example, approximately45°, outer angles with the surface of the multilayer circuit board 2 atfor example both end portions. Note that an outer angle of the opticalwaveguide path 11 formed with the surface of the multilayer circuitboard 2 means the outer angle of the closed shape formed by thecross-section of the optical waveguide path 11 along the direction oflight propagation. Here, the upper cladding layer 53 corresponds to aconcrete example of the “first cladding layer” of the present invention,while the lower cladding layer 55 corresponds to a concrete example ofthe “second cladding layer” of the present invention. Further, theinclined surfaces 11 a and 11 b correspond to concrete examples of the“light reflection portion” of the present invention.

The inclined surfaces 11 a and 11 b give outer angles together with thesurface of the multilayer circuit board 2 of approximately 45° asmentioned above and function to reflect a light signal striking them ina direction orthogonal to the main surface of the multilayer circuitboard 2 from the outside of the optical waveguide path 11 so as tointroduce it into the optical waveguide path 11 and, at the same time,reflect a light signal propagated in the optical waveguide path 11 so asto guide it to a direction orthogonal to the main surface of themultilayer circuit board 2.

The upper cladding layer 53 and the lower cladding layer 55 areconstituted by an epoxy resin comprised mainly of bisphenol having arefractive index of for example about 1.52 and has a thickness of forexample 20 μm. Further, the core layer 54 is constituted by a materialhaving a larger refractive index than that of the material constitutingthe upper cladding layer 53 and the lower cladding layer 55, forexample, an epoxy resin having a refractive index of about 1.54. Thethickness of the core layer 54 is for example 20 μm, while the width isfor example 60 μm. Note that the upper cladding layer 53, core layer 54,and lower cladding layer 55 can be constituted by other materials too,for example, a polyimide, polymethyl methacrylate (PMMA), or otheracrylic resin, polyethylene, polystyrene, or other polyolefin resin, orsynthetic rubber so far as they satisfy the condition that therefractive index of the core layer 54 be larger than the refractiveindex of the upper cladding layer 53 and the lower cladding layer 55.

The light receiving element 21 converts the light signal striking itsincident surface from the optical waveguide path to an electrical signalwhich it then outputs to the electrical wiring 3 formed on themultilayer circuit board 2. A photodiode can be mentioned as an examplethereof. Here, the incident surface of the light receiving element 21 isprovided at a position facing the inclined surface 11 a of the opticalwaveguide path 11 so as to be orthogonal to the main surface of themultilayer circuit board 2.

The light emitting element 31 converts an electrical signal input to thelight emitting element 31 through the electric wiring 3 of themultilayer circuit board 2 to a light signal which it then emits to theinclined surface 11 b of the optical waveguide path 11. A light emittingdiode (LED) can be mentioned as an example thereof. Note that the lightemitting surface of the light emitting element 31 is provided at aposition facing the inclined surface 11 b of the optical waveguide path11 so as to be orthogonal to the main surface of the multilayer circuitboard 2.

In the IC chips 25 and 35, for example, signal processing circuits,memory circuits, and other electronic circuits are integrated. Electricpower is supplied through the electrical wiring 3 of the multilayercircuit board 2. Further, these IC chips 25 and 35 are electricallyconnected to the light receiving element 21 and the light emittingelement 31 by the electric wiring 3 and function to transferringelectrical signals between the light receiving element 21 and the lightemitting element 31.

The light receiving element 21, light emitting element 31, and IC chips25 and 35 are provided with for example electrode pads (notillustrated). These electrode pads contact ball bumps (projections) BPmade of for example a solder (Pb—Sn solder) containing as principalcomponents lead (Pb) and tin (Sn). Namely, the light receiving element21 and light emitting element 31 and the IC chips 25 and 35 are arrangedsandwiching the optical waveguide path 11 therebetween and, at the sametime, electrically connected to the electric wiring 3 on the multilayercircuit board 2 by the bumps BP. The bumps BP correspond to concreteexamples of the “connection electrode” of the present invention. Notethat, as the bumps BP, use can be also made of ball bumps made of gold(Au) or ones obtained by coating a solder on spherical bodies (ballcores) having electric conductivity such as copper (Cu).

Next, an explanation will be made of the mode of operation of thisoptical waveguide 1.

In this optical waveguide 1, the electric power supplied from forexample the electric wiring 3 of the multilayer circuit board 2 placesthe light receiving element 21, light emitting element 31, and IC chips25 and 35 in an operation ready state. In this state, when an electricalsignal is output from the IC chip 35 to the light emitting element 31,the light emitting element 31 converts the electrical signal to a lightsignal which it then emits from the emitting surface. The emitted lightsignal strikes the inclined surface 11 b where it is reflected and thenintroduced into the optical waveguide path 11. This light signal is thenpropagated in the optical waveguide path 11 and reflected at theinclined surface 11 a to strike the incident surface of the lightreceiving element 21. The light signal striking the light receivingelement 21 is converted to an electrical signal which is input to the ICchip 25. In this way, the light signal is transmitted at a high speedbetween the IC chip 35 and the IC chip 25. Further, a low speed controlsignal or other signal which can be transmitted at a relatively lowspeed is transmitted as an electrical signal as it is by the electricalwiring 3 of the multilayer circuit board 2. Here, the transmission rateof the light signal corresponds to a concrete example of the “firstrate” of the present invention, while the transmission rate of theelectrical signal corresponds to a concrete example of the “second rate”of the present invention.

In this way, in the optical waveguide according to the presentembodiment, electric power can be supplied to the IC chips 25 and 35,the light receiving element 21, and the light emitting element 31 andthe signals which control the IC chip 35 and the IC chip 25 and can betransmitted at a relatively low speed can be transmitted by the electricwiring 3 of the multilayer circuit board 2, while signals which must betransmitted between the IC chip 35 and the IC chip 25 at a high speedcan be transmitted as light signals by the light emitting element 31,the optical waveguide path 11, and the light receiving element 21.Accordingly, the disadvantage of the delay of the signal due to the CRtime constant of the electric wiring 3 can be solved. Further, it isalso possible to solve the problem of the electromagnetic radiationnoise and the disadvantage of malfunctions due to the disturbances inthe waveform.

Further, in the optical waveguide according to the present embodiment,the IC chips 25 and 35, the light receiving element 21, and the lightemitting element 31 are mounted on the multilayer circuit board 2through the optical waveguide path 11 and the bumps BP, so the opticalwaveguide path 11 supports the IC chips 25 and 35, the light receivingelement 21, and the light emitting element 31, and functions as a spacerfor securing a space for arranging the bumps BP. For this reason, the ICchips 25 and 35, the light receiving element 21, and the light emittingelement 31 can be stably secured to the multilayer circuit board 2.Further, the optical waveguide path 11 enables the distance between theIC chips 25 and 35, the light receiving element 21, and the lightemitting element 31 and the electric wiring 3 to be maintained at apredetermined value.

Next, an explanation will be made of the method for producing theoptical waveguide according to the present embodiment by referring toFIG. 2 through FIG. 12. Note that FIG. 2 through FIG. 4 are perspectiveviews each representing one step of production. FIG. 5A is a perspectiveview of a step of production, while FIG. 5B shows the sectionalstructure along a line VB-VB of FIG. 5A. FIG. 6, FIG. 7, and FIG. 12 aresectional views each representing one step of production. FIGS. 8A, 9A,and 11A are partially cutaway perspective views each representing onestep of production, while FIGS. 8B, 9B, and 11B show the sectionalstructure taken along the lines VIIIB-VIIIB, IXB-IXB, and XIB-XIB,respectively.

First, as shown in FIG. 2, a transparent substrate 51 excellent inflatness made of for example quartz glass, lead glass, soda glass, ormica or another material transmitting light in a range from theultraviolet zone to visible zone relatively well (light transmittingmaterial) is prepared. A substrate separation (isolation) layer 52 madefor example of silicon dioxide (SiO₂) having a thickness of 500 nm isformed on this transparent substrate 51 by for example plasma chemicalvapor deposition (CVD) thermal CVD, optical CVD, or another process.This substrate separation layer 52 is for separating the transparentsubstrate 51 from the optical waveguide path 11. Details thereof will bementioned later. Note that silicon dioxide is a substantiallytransparent material with respect to light in the range from theultraviolet zone to the visible zone. Next, an epoxy resin is coated onthe substrate separation layer 52 to give a thickness of about 20 μm byfor example spin coating, then heat treatment is carried out to causethe resin to solidify and thereby form an upper cladding layer 53 havinga refractive index of for example 1.52.

Next, as shown in FIG. 3, a method similar to the method for formationof for example the upper cladding layer 53 and a material having ahigher refractive index than that of the material constituting the uppercladding layer 53 (for example epoxy resin) are used to form a corelayer 54′ which has for example a refractive index of 1.54 and thicknessof about 30 μm on the upper cladding layer 53.

Next, as shown in FIG. 4, a photoresist film (not illustrated) having apredetermined pattern is formed, then, for example, RIE is carried outby using this photoresist film as a mask. By this, the core layer 54′becomes a plurality of stripes of core layers 54 spaced from each other.

Next, as shown in FIGS. 5A and 5B, a method similar to the method forforming the upper cladding layer 53 and an identical material to thatfor the upper cladding layer 53 are used to form a lower cladding layer55 having a thickness of about 20 μm on the entire surface of thetransparent substrate 51.

Note that the upper cladding layer 53, core layer 54, and the lowercladding layer 55 may also be formed by coating a photo-curing resin onthe respective underlying layers and then curing the resin byirradiating this photo-curing resin with light.

Next, as shown in FIG. 6, a photoresist film 56 having for example athickness of about several tens of Am is formed on the lower claddinglayer 55. Next, a predetermined exposure processing and developmentprocessing are applied to the photoresist film 56 to process thephotoresist film 56 to a predetermined pattern, then this patternedphotoresist film 56 is heat treated at a temperature more than forexample the glass transition temperature. As a result, an edge part ofthe photoresist film 56 becomes fluid and the inclined surfaces (Eportions of FIG. 6) are formed.

Next, as shown in FIG. 7, the optical waveguide path 11 isanisotropically etched by using for example an RIE system orelectro-cyclotron resonance (ECR) system with the photoresist film 56 asa mask. By this, both end portions of the optical waveguide path 11 areformed with shapes corresponding to the inclined surfaces of thephotoresist film 56, that is, inclined surfaces 11 a and 11 b givingouter angles with the surface of the transparent substrate 51 ofapproximately 135°. Thereafter, the photoresist film 56 is removed.

Next, as shown in FIGS. 8A and 8B, for example, a multilayer circuitboard 2 is prepared. The desired region of this multilayer circuit board2 is formed with a bonding layer 6 having a thickness of about 10 μmmade of a glass epoxy resin or other photo-curing resin by for examplespin coating, dip coating, spraying, printing, or another process.

Next, as shown in FIG. 9, the transparent substrate 51 on which theoptical waveguide path 11 is formed is turned upside down, and theoptical waveguide path 11 is positioned with and pressed against themultilayer circuit board 2 on which the bonding layer 6 is formed. Then,in a state where the transparent substrate 51 side of the opticalwaveguide path 11 is pressed against the multilayer circuit board 2,light L is irradiated from the transparent substrate 51 side toward themultilayer circuit board 2 side. By this, the photo-curing resinconstituting the bonding layer 6 cures and the multilayer circuit board2 is secured to the desired position of the optical waveguide path 11.If a large amount of light L is irradiated for a short time, distortionwill be created in the optical waveguide path 11 and the lightpropagation loss will end up becoming large. Therefore, a relativelysmall amount of the light L is irradiated over a long time. For example,when using an extra-high pressure mercury lamp (wavelength: center ofg-rays (436 nm)) is used, light is irradiated for 5 minutes with anoutput of 10 mW/cm².

FIG. 10 is a graph showing the light transmission rate of the epoxyresin (thickness: 1 mm) used as the core layer 54, upper cladding layer53, and the lower cladding layer 55 in the present embodiment. Theordinate indicates the light transmission rate (unit: %), while theabscissa indicates the wavelength of the light (unit: nm). As seen alsofrom FIG. 10, this epoxy resin is a light transmitting resin whichallows the light in the near ultraviolet zone and visible zone having alonger wavelength than about 350 nm to pass therethrough at a rate ofabout 90%. Further, as described above, the transparent substrate 51 andthe substrate separation layer 52 have a sufficient transparency fromthe ultraviolet zone to the visible zone. Accordingly the light Lemitted from for example the extra-high pressure mercury lamp passesthrough the transparent substrate 51, substrate separation layer 52, andthe optical waveguide path 11 and sufficiently reaches the bonding layer6, whereby the bonding layer 6 made of for example glass epoxy resin iscompletely cured.

Next, in the state where the multilayer circuit board 2 is secured tothe optical waveguide path 11, the transparent substrate 51 and themultilayer circuit board 2 are dipped in for example a dilutehydrofluoric acid (HF) solution or a buffered hydrofluoric acid (BHF)solution. Due to this, as shown in FIGS. 11A and 11B, the substrateseparation layer 52 formed between the transparent substrate 51 and theoptical waveguide path 11 is dissolved and removed, and the state wherethe transparent substrate 51 on the substrate separation layer 52 isseparated from the optical waveguide path 11 (lifted off) is exhibited,whereby the optical waveguide path 11 is transferred to the multilayercircuit board 2. Thereafter, the multilayer circuit board 2 to which theoptical waveguide path 11 is transferred is pre-washed by water, washed,and dried.

Next, as shown in FIG. 12, the bumps BP are formed on the notillustrated electrode pads of the light emitting element 31, the ICchips 25 and 35, and the light receiving element 21. Specifically, forexample, spherical portions made of gold formed at the tips of fine goldwires are pressed against the electrode pads of the light receivingelement 21, light emitting element 31, and the IC chips 25 and 35, thenthe fine wires are pulled away from the spherical portions to cut thespherical portions from the fine wires and form the bumps BP.Thereafter, the light receiving element 21, light emitting element 31,and the IC chips 25 and 35 are mounted on the multilayer circuit board 2by flip chip bonding and reflow is performed to an extent not damagingthe optical waveguide path 11. Here, flip chip bonding means the processof positioning the bumps BP closely against the electrodes (wiring)facing the bumps BP provided on the multilayer circuit board 2 andapplying heat and pressure to bond them. Note that it is also possibleto perform the reflow by mounting the light receiving element 21 etc.after applying solder paste to the desired positions on the multilayercircuit board 2 without use of flip chip bonding by for example byprinting. Further, it is also possible to mount for example chip typeresistors, capacitors, inductors, and other elements besides the lightreceiving element 21, light emitting element 31, and the IC chips 25 and35.

Finally, although not illustrated, a sealing resin (for example epoxyresin) is introduced between the mounted light receiving element 21,light emitting element 31,.and the IC chips 25 and 35 and the multilayercircuit board 2 by utilizing for example the capillary action so as toseal the light receiving element 21, light emitting element 31, and theIC chips 25 and 35. By this, the reliably of the connection between thelight receiving element 21, light emitting element 31, and the IC chips25 and 35 with the electric wiring 3 is improved.

In this way, in the method for producing an optical waveguide accordingto the present embodiment, an optical waveguide path 11 is formed inadvance on a transparent substrate 51 excellent in flatness and thentransferred to the multilayer circuit board 2, therefore even in a casewhere a multilayer circuit board 2 having a large surface unevenness isused as the supporting base, it is possible to prepare an opticalwaveguide 1 having an optical waveguide path 11 having little lightpropagation loss.

Further, according to the present embodiment, a relatively low costceramic multilayer circuit board or printed circuit board is used as themultilayer circuit board 2 and parts such as the IC chips 25 and 35, thelight receiving element 21, and the light emitting element 31 aremounted on this multilayer circuit board 2, therefore the parts can beeasily mounted. Further, in comparison with the method of the relatedart of forming the electrical wiring layer on a silicon substrate bythin film technology and mounting the parts thereon, it is possible toinexpensively produce an optical waveguide 1 which can simultaneouslytransmit signals by light, transmit signals by electricity, and supplyelectric power.

Further, according to the present embodiment, the method is divided to athin film process for forming the optical waveguide path 11 and a thickfilm process for forming the multilayer circuit board 2, and the opticalwaveguide path 11 formed by the thin film process is transferred to themultilayer circuit board 2, therefore there is no need to use spincoating, by which it is considered difficult to form a film on asubstrate other than a disk-like substrate, and therefore the opticalwaveguide path 11 can be easily formed on a substrate of a polygonal orother shape. As a result, the degree of freedom of selection of theshape and the material etc. of the substrate serving as the supportingbase of the optical waveguide path 11 (here the multilayer circuit board2) is increased and the manufacturing cost can be reduced.

Further, when using the process of forming the optical waveguide path 11in advance on the transparent substrate 51 separately from themultilayer circuit board 2 and transferring the optical waveguide path11 as in the present embodiment and at the same time forming thesectional shape (particularly the width) of the optical waveguide path11 large, the positioning of the IC chips 25 and 35, light emittingelement 21, and the light receiving element 31 with respect to theoptical waveguide path 11 becomes easy and it becomes possible to lowerthe manufacturing cost in this point as well.

Note that, in the present embodiment, the IC chips 25 and 35, lightemitting element 21, and the light receiving element 31 were mounted incontact with the optical waveguide path 11 using the optical waveguidepath 11 as a spacer, but as mentioned above, it is necessary to takecare so as not to damage the optical waveguide path 11 when performingthe reflow at the time of mounting. Accordingly, it is also possible tomount them so that the optical waveguide path 11 and the IC chips 25 and35, light emitting element 21, and the light receiving element 31 arespaced from each other using the bumps BP as spacers. In this case, thebumps BP enable the distance between the IC chips 25 and 35, lightemitting element 21, and the light receiving element 31 and the electricwiring 3 to be maintained at the predetermined value and, at the sametime, enable electrical connection with a high reliability.

Second Embodiment

The second embodiment relates to a method for producing the opticalwaveguide. The optical waveguide covered by this method is similar tothat of the first embodiment except for the point that the opticalwaveguide paths are spaced from each other. Below, an explanation willbe made of the method for producing the optical waveguide of the presentembodiment by referring to FIG. 1 through FIGS. 5A and 5B and FIG. 13through FIG. 20. FIG. 13 through FIG. 20 are partially cutawayperspective views representing steps of production. Note thatconstituent elements the same as those of the first embodiment are giventhe same reference numerals and detailed explanations thereof areomitted.

In the method of production according to the present embodiment, first,in the same way as the steps of productions shown in FIG. 2 to FIGS. 5Ato 5B of the first embodiment, the substrate separation layer 52 and theoptical waveguide path 11 are formed on a transparent substrate 51.

Next, as shown in FIG. 13, for example-plasma CVD, thermal CVD, opticalCVD, or another process is used to form a peeling layer 91 made ofsilicon dioxide (SiO₂) having for example a thickness of 500 nm on theoptical waveguide path 11. This peeling layer 91 is for selectivelyremoving a light blocking film 92 mentioned later (refer to FIG. 15).Then, using photolithography in the same way as the first embodiment,both end portions of the optical waveguide path 11 are formed withinclined surfaces 11 a and 11 b (not illustrated here) giving an outerangle with the surface of the transparent substrate 51 of approximately135°.

Next, as shown in FIG. 14, for example a not illustrated photoresistfilm having a predetermined pattern is formed. Using this photoresistfilm as a mask, laser processing or plasma etching using for exampleoxygen (O) plasma, ion beam etching, etching using powder (powder beametching), or other etching is performed so as to selectively remove thepeeling layer 91, optical waveguide path 11, and the substrateseparation layer 52. Here, it is necessary to reliably remove them up tothe lower surface of the optical waveguide path 11 (that is, theboundary of the upper cladding layer 53 and the substrate separationlayer 52), therefore the laser-processing or etching must be carried outuntil the substrate separation layer 52 is exposed. By this, the opticalwaveguide path 11 and the peeling layer 91 formed on this are dividedinto a plurality of stripes of optical waveguide paths 11′ and peelinglayers 91′ spaced from each other. Note that, where the part to beremoved is a simple pattern constituted by straight lines, it can beremoved by dicing or the like too.

Next, as shown in FIG. 15, the light blocking film 92 made of chromium(Cr) of for example a thickness of 100 nm is formed on the entireexposed surfaces of the transparent substrate 51, optical waveguide path11, and the peeling layer 91 by for example vapor deposition orsputtering. Note that the light blocking film 92 is for selectivelyblocking the light when exposing the bonding layer 6 mentioned later tocure it. Incidentally, as the material constituting the light blockingfilm 92, materials other than chromium can be used too so far as theyare materials which can block light. Specifically use can be made ofaluminum, tantalum (Ta), etc. too.

Next, as shown in FIG. 16, the peeling layers 91′ made of the silicondioxide are dissolved and removed by using for example a dilutehydrofluoric acid solution and thereby selectively remove the lightblocking film 92 in contact with the peeling layers 91′ (that is, on theoptical waveguide paths 11′) (lift-off process). By this, a state whereonly the upper surfaces of the optical waveguide paths 11′ are exposedis exhibited.

Next, as shown in FIG. 17, a multilayer circuit board 2 is prepared. Adesired region on this multilayer circuit board 2 is formed with abonding layer-6 having a thickness of about 10 μm made of a glass epoxyresin or other photo-curing resin by for example spin coating, dipcoating, spraying, printing, or another process.

Next, as shown in FIG. 18, for example, in the same way as the step ofproduction shown in FIGS. 9A and 9B of the first embodiment, thetransparent substrate 51 on which the optical waveguide paths 11′ areformed is turned upside down, the multilayer circuit board 2 is pressedagainst the optical waveguide paths 11′, and light L is irradiated fromthe side of the transparent substrate 51 toward the direction of themultilayer circuit board 2. Here, the light blocking film 92 is notformed at the boundaries between the optical waveguide paths 11′ and thebonding layer 6, but is formed only at the side surfaces of the opticalwaveguide paths 11′ and the regions of the transparent substrate 51facing the multilayer circuit board 2 where the optical waveguide paths11′ are not formed. Accordingly, the light L irradiated from thetransparent substrate 51 side reaches the bonding layer 6 only in theregions A in which the optical waveguide paths 11′ are formed and isblocked by the light blocking film 92 and does not reach the bondinglayer 6 in another regions B. As a result, only the regions A of thebonding layer 6 under the optical waveguide paths 11′ are cured, whilethe other regions B are left uncured. Then, by this cured bonding layer6, the multilayer circuit board 2 is secured to the optical waveguidepaths 11′.

Next, as shown in FIG. 19, the resin of the regions B to which the lightL is not irradiated due to the light blocking film 92 and which is leftuncured is selectively dissolved and removed by for example acetone orethanol.

Next, as shown in FIG. 20, the light blocking film 92 made of chromiumis dissolved by using for example an acidic solution (for examplehydrochloric acid solution), then the substrate separation layer 52 isdissolved and removed by for example a process similar to the process ofthe step shown in FIGS. 11A and 11B of the first embodiment, whereby theoptical waveguide paths 11′ are transferred to the multilayer circuitboard 2. The following steps are identical to those of the firstembodiment.

In this way, in the present embodiment, a plurality of optical waveguidepaths 11′ separated from each other are formed on the transparentsubstrate 51 in advance and, at the same time, the uncured bonding layer6 formed on the entire surface of the multilayer circuit board 2 and theoptical waveguide paths 11′ pressed against each other and only thebonding layer 6 under the optical waveguide paths 11′ forming region isselectively cured by using the light blocking film 92, therefore theplurality of optical waveguide paths 11′ on the transparent substrate 51can be transferred to the multilayer circuit board 2 well. Namely, aplurality of optical waveguide paths 11′ which are formed spaced fromeach other and have little light propagation loss can be arranged on themultilayer circuit board 2.

Further, here, the explanation was made of the case of forming aplurality of optical waveguide paths 11′ of stripe shapes, but anyshapes of optical waveguide paths 11′ (for example, L-shapes, U-shapes,or arcs) formed on the transparent substrate 51 can be transferred tothe multilayer circuit board 2 when using the method of production ofthe present embodiment. Also, the optical waveguide paths can betransferred to only required portions without transferring them toregions to which optical-waveguide paths should not be transferred. forexample, the electrode forming regions of the multilayer circuit board2.

Third Embodiment

The third embodiment of the present invention relates to a method forproducing a optical waveguide path. The structure of the opticalwaveguide covered by the method is similar to that shown in FIG. 1. Themethod for producing the optical waveguide according to the presentembodiment is similar to that of the first embodiment except for thepoint that the method for forming the inclined surfaces 11 a and 11 b ofthe optical waveguide path 11 is different. Below, the explanation willbe made by referring to FIG. 21 through FIG. 23. Note that constituentelements the same as those of the first embodiment are given the samereference numerals and detailed explanations thereof are omitted.

In the method for forming the inclined surfaces of the presentembodiment, first, as shown in FIG. 21, an optical waveguide path 11comprising the upper cladding layer 53, core layer 54, and the lowercladding layer 55 is formed on a transparent substrate 51 via thesubstrate separation layer 52. Next, a metal film 60 made of for examplealuminum is formed by vapor deposition on the lower cladding layer 55.Next, a photoresist film 56 is formed on the metal film 60, then thephotoresist film 56 is exposed and developed to form the desiredphotoresist pattern. Further, the photoresist film 56 is heat treated ata temperature more than for example the glass transition temperature,whereby the inclined surfaces E are formed at the edge parts of thephotoresist film 56.

Next, as shown in FIG. 22, the metal film 60 is etched with thephotoresist film 56 as the mask so as to process the two end portions ofthe metal film 60 to inclined surfaces F having shapes corresponding tothe inclined surfaces E of the photoresist film 56.

Next, as shown in FIG. 22, for example a CO₂ gas laser or other laserbeam LB is fired from a desired angle using the metal film 60 as themask so as to cut the optical waveguide path 11. At this time, the cutsurfaces which become the end portions of the optical waveguide path 11are made to form inclined surfaces 11 a and 11 b inclined approximately45° with respect to the longitudinal direction (light propagationdirection) of the optical waveguide path 11. The following steps aresimilar to those of the first embodiment.

In this way, according to the present embodiment, the inclined surfaces11 a and 11 b of the end portions of the optical waveguide path 11 areformed by cutting the optical waveguide path 11 by firing the laser beamLB from a predetermined angle, therefore they can be formed moreprecisely and easily than the case where the inclined surfaces 11 a and11 b are formed by only the thin film formation process.

Fourth Embodiment

The fourth embodiment of the present invention relates to a method forproducing an optical waveguide path. The structure of the opticalwaveguide covered by the method is similar to that shown in FIG. 1. Themethod for producing the optical waveguide according to the presentembodiment is similar to that of the first embodiment except for thepoint that the method for forming the inclined surfaces 11 a and 11 b ofthe optical waveguide path 11 is different. Below, the explanation willbe made by referring to FIG. 24 through FIG. 27. Note that constituentelements the same as those of the first embodiment are given the samereference numerals and detailed explanations thereof are omitted.

In the present embodiment, first, as shown in FIG. 24, a process similarto that for the first embodiment is used to form a substrate separationlayer 52 on a transparent substrate 51 and to form an optical waveguidepath 11 comprising the upper cladding layer 53, core layer 54, and thelower cladding layer 55 on this.

Next, as shown in FIG. 25, a heat tool T having two chamfered slopes Tainclined approximately 45° with respect to the tip surface Tb isprepared. This heat tool T is heated to a temperature more than theglass transition temperature of the resin material constituting theupper cladding layer 53, core layer 54, and the lower cladding layer 55constituting the optical waveguide path 11. Then, the tip of the heatedheat tool T is pressed against the multilayer circuit board 2 to meltthe optical waveguide path 11. If necessary, the heat tool T may bemoved in a direction along the main surface of the multilayer circuitboard 2.

Next, as shown in FIG. 26, the heat tool T is removed from themultilayer circuit board 2, whereby a depression H is formed in theregion in which the optical waveguide path 11 is melted. Thus, the innersurfaces of the depression H become the inclined surfaces 11 a and 11 binclined approximately 45° with respect to the longitudinal direction ofthe optical waveguide path 11. Note that scum K is produced around thedepression H due to heat deformation. For this reason, the surface ofthe substrate 51 where the scum K is formed is lightly polished bypressing it against the polishing surface of a grinding substrate 71. Bythis polishing, as shown in FIG. 27, the scum K around the depression His removed and, at the same time, the upper surface of the opticalwaveguide path 11 is flattened. Finally, washing and drying treatmentare performed, whereby the inclined surfaces 11 a and 11 b of theoptical waveguide path 11 are completed. The following steps are similarto those of the first embodiment.

In this way, according to the present embodiment, the inclined surfaces11 a and 11 b of the optical waveguide path 11 are formed by the slopesTa of the heat tool T, therefore these inclined surfaces 11 a and 11 bcan be formed at an identical inclination angle to that of the slopes Taof the heat tool T and the precision of the inclined surfaces 11 a and11 b can be raised.

Fifth Embodiment

The fifth embodiment of the present invention relates to a method forproducing an optical waveguide path. The structure of the opticalwaveguide covered by the method is similar to that shown in FIG. 1. Themethod for producing the optical waveguide according to the presentembodiment is similar to that of the first embodiment except for thepoint that the method for forming the inclined surfaces 11 a and 11 b ofthe optical waveguide path 11 is different. Below, the explanation willbe made by referring to FIG. 28 through FIG. 30. Note that constituentelements the same as those of the first embodiment are given the samereference numerals and detailed explanations thereof are omitted.

In the present embodiment, first, as shown in FIG. 28, a process similarto that for the first embodiment is used to form a substrate separationlayer 52 on a transparent substrate 51 and to form an optical waveguidepath 11 comprising the upper cladding layer 53, core layer 54, and thelower cladding layer 55 on this.

Next, as shown in FIG. 29, the transparent substrate 51 on which theoptical waveguide path 11 is formed is cut at an angle of 45° withrespect to the longitudinal direction of the optical waveguide path 11(light propagation direction) by for example a dicing saw or othercutting means (F portion of FIG. 29).

Next, as shown in FIG. 30, the cut transparent substrate 51 is inclinedat an angle of 45° with respect to a grinding substrate 81, then the cutsurface of the transparent substrate 51 brought into contact with thegrinding surface of the grind substrate 81 to grind it. By this, aninclined surface 11 a inclined 45° with respect to the longitudinaldirection of the optical waveguide path 11 can be formed at one endportion of the optical waveguide path 11. The following steps aresimilar to those of the first embodiment.

In this way, according to the present embodiment, the inclined surface11 a of the optical waveguide path 11 is mechanically ground, thereforethe precision of the inclined surface 11 a can be raised. Note that themethod for forming this inclined surface 11 a is convenient since endmachining of a plurality of optical waveguide paths 11 arranged alongthe main surface of a transparent substrate 51 can be carried outtogether.

Sixth Embodiment

The sixth embodiment of the present invention relates to an opticalwaveguide constituted as an optical module for performing the transferof data through a single-core optical fiber. FIG. 31 is a view of thesectional structure of the optical waveguide according to the presentembodiment. Note that constituent elements the same as those of thefirst embodiment are given the same reference numerals and detailedexplanations thereof are omitted.

This optical waveguide is provided with a multilayer circuit board 2(for example, a glass epoxy multilayer circuit board), an opticalwaveguide path 11 formed on the multilayer circuit board 2, an end faceemitting type laser diode 101 as the light emitting element fortransmission and an IC chip 102 mounted on the multilayer circuit board2 via the bumps BP, a light receiving element 103 embedded in themultilayer circuit board 2, and a single-core optical fiber 201 arrangedat a position facing the light receiving element 103 on the multilayercircuit board 2. Further, the drive circuit of the laser diode 101 and atrans-impedance amplifier and other receiving circuits and othercircuits and parts can be mounted (not illustrated) on the multilayercircuit board 2 as well.

The optical waveguide path 11 is transferred onto the multilayer circuitboard 2 by the method of the first embodiment and secured to themultilayer circuit board 2 by a bonding layer 6 serving also as apassivation film made of resin. One end of the optical waveguide path 11is formed with an inclined surface 11 a giving an outer angle with thesurface of the multilayer circuit board 2 of approximately 45° andserving as a reflection surface. The optical waveguide path 11 ispositioned so that the inclined surface 11 a is positioned at the centerportion of the light receiving element 103. The light receiving element103 has a sensitivity with respect to the wavelength of the receivedlight.

The single-core optical fiber 201 is constituted by a fiber core layer202 formed inside and a fiber cladding layer 203 formed on the outercircumference of the fiber core layer 202 and is provided so that oneend portion is spaced exactly a predetermined distance from the lightreceiving element 103. The diameter of the fiber core layer 202 is madesufficiently large in comparison with the size of the inclined surface11 a.

In this optical waveguide, a light signal output from the laser diode101 is input to the optical waveguide path 11, propagated through theoptical waveguide path 11, and reflected at the inclined surface 11 aand introduced into the single-core optical fiber 201 as the emittedlight Lout with a good efficiency. Further, the incident light Linoutput from the single-core optical fiber 201 is irradiated over a wideregion of the multilayer circuit board 2 including the inclined surface11 a of the optical waveguide path 11, but the amount of the lightbounced back at the inclined surface 11 a is very small. Most of theincident light Lin is absorbed into a light receiving portion 103 a ofthe light receiving element 103, therefore it is possible to perform thereception with a good efficiency.

In this way, according to the present embodiment, the degree of freedomof the arrangement between the single-core optical fiber 201, lightreceiving element 103, and the laser diode 101 becomes large and it ispossible to obtain an optical module at a low cost and with a goodefficiency.

While the present invention was explained above by giving preferredembodiments, the present invention is not limited to the embodiments andcan be modified in various ways. For example, in the embodiments, astructure in which the lower cladding layer 55 of the optical waveguidepath 11 was bonded onto the bonding layer 6 was adopted, but if thebonding layer 6 is constituted by a material achieving the same functionas that of the lower cladding layer 55, a structure in which the lowercladding layer 55 acts also as the bonding layer 6 can be obtained.Further, for the upper cladding layer 53, when transferring the opticalwaveguide path 11 to the multilayer circuit board 2, it is also possibleto obtain a structure utilizing air as the cladding layer and thereforein which the upper cladding layer 53 is not formed. As a result, theprocess of formation of the optical waveguide path 11 can be simplified,and the manufacturing cost can be lowered.

Further, in the first embodiment, the explanation was made of the casewhere a photo-curing resin was used as the material for forming thebonding layer 6, but it is also possible to use a heat-curing resin. Inthis case, the optical waveguide path 11 is formed on the transparentsubstrate 51, then a bonding layer 6 made of a heat-curing resin such asan epoxy resin or acrylic resin is formed at the desired region on themultilayer circuit board 2 by for example printing and is heated in thestate where the multilayer circuit board 2 is pressed against theoptical waveguide path 11 on the transparent substrate 51. By this, theheat-curing resin constituting the bonding layer 6 is cured, and themultilayer circuit board 2 is secured to the optical waveguide path 11.In this case, the substrate for forming the optical waveguide path(first substrate in the present invention) need not be a lighttransmitting type. Note that to facilitate the positioning at the timeof transfer, however, preferably a transparent substrate is used.

Further, in the embodiments, silicon dioxide was used as the materialfor forming the substrate separation layer 52 and, at the same time,this substrate separation layer 52 was dissolved and removed by dippingthis in a dilute hydrofluoric acid solution or buffered hydrofluoricacid solution, but it is also possible to dissolve the substrateseparation layer 52 by spraying an organic solvent such as acetone orisopropyl alcohol and then separate the substrate separation layer 52and the optical waveguide path 11 at the boundary thereof. Further, itis also possible to form the substrate separation layer 52 by aluminum(Al) or copper or another metal material which can be wet etched and todissolve and remove this substrate separation layer 52 by using dilutehydrochloric acid (HCl), sodium hydroxide (NaOH), potassium hydroxide(KOH), or another solution.

Further, in the embodiments, quartz glass or another light transmittingmaterial was used as the transparent substrate 51, but it is alsopossible to use a photosensitive glass or other material which has alight transmitting property and can be dissolved. In this case, thephotosensitive glass is dissolved when dipping the same in a 5 volt orless dilute hydrofluoric acid solution, therefore the substrateseparation layer 52 becomes unnecessary.

Further, in the embodiments, the light reflecting portions (inclinedsurfaces 11 a, 11 b) were provided on the entire end surfaces of theoptical waveguide path 11, but they may also be provided only on the endsurfaces of the core layer 54.

Summarizing the effects of the present invention, as explained above,according to the optical waveguide of one aspect of the presentinvention, since the optical waveguide path is separately formed inadvance and then this optical waveguide path is arranged on thesubstrate and secured, unlike the case where the optical waveguide pathis directly formed on the substrate, it becomes possible to secureconstant characteristics being influenced by the type and shape of thesubstrate supporting the optical waveguide path. Accordingly, there isthe effect that the optical waveguide it can be constituted by using anysubstrate.

Particularly, according to the optical waveguide of another aspect ofthe present invention, since at least one of the optical waveguide path,light emitting element, and the light receiving element and theintegrated circuits are provided on a substrate formed with theelectrical wiring, it is possible to selectively use the transmissionformat in accordance with the purpose and object, for example,transmitting a signal which has to be transmitted at a high speed as alight signal, while transmitting a signal which can be transmitted at arelatively low speed as an electrical signal. For this reason, thepropagation delay of the signal, which becomes the disadvantage when asignal is transmitted by electrical wiring, is eliminated and, at thesame time, the influence of electromagnetic noise becomes small and, asa result, malfunctions due to disturbances in the waveform can beeffectively prevented. Accordingly, use of this optical waveguideenables the transfer of signals at a high speed, the realization ofwhich has been difficult by only electrical wiring, and enables a greatimprovement in the performances of the system and the network.

Further, according to the method for producing an optical waveguide ofstill another aspect of the present invention, since the opticalwaveguide path formed on the first substrate is transferred to thesecond substrate, there is the effect that an optical waveguide pathwhich could be formed only on an expensive substrate excellent in theheat resistance in the related art can be formed on a cheaper substratemade of any material and having any shape. Further, by using a substrateexcellent in flatness as the first substrate, an optical waveguide pathwith little light propagation loss can be obtained.

Particularly, according to the method for producing an optical waveguideof still another aspect of the present invention, since a plurality ofoptical waveguide paths spaced from each other are formed on the firstsubstrate, the second substrate is pressed against these opticalwaveguide paths via the bonding layer, and only the regions in thebonding layer corresponding to the regions in which the opticalwaveguide paths are formed are selectively exposed and cured, aplurality of optical waveguide paths spaced from each other can betransferred to the desired regions of any substrate.

Further, according to the method for producing an optical waveguide ofstill another aspect of the present invention, since an electricalwiring board formed in advance by a thick film process is used as thesecond substrate, the optical waveguide can be produced more easily andcheaply than the case where the electrical wiring is formed on asubstrate by using thin film technology and then the optical waveguidepath is formed on this.

Particularly, according to the method for producing an optical waveguideof still another aspect of the present invention, since, as the materialfor forming the first and second cladding layers, one containing atleast one material selected from a polyimide, epoxy resin, acrylicresin, polyolefin resin, and synthetic rubber cheaper than quartz isused, the cost of the materials of the optical waveguide path can bereduced.

Further, according to the method for producing an optical waveguide ofstill another aspect of the present invention, since the inclinedsurface of at least one end portion of the optical waveguide path isformed by irradiating a laser beam at a predetermined angle to cut theoptical waveguide path, the inclined surface can be more precisely andeasily formed than the case where it is formed by only a thin filmformation process.

Further, according to the method for producing an optical waveguide ofstill another aspect of the present invention, since at least one endsurface of the optical waveguide path is processed to an inclinedsurface by heating a heat tool having an inclined surface of apredetermined angle at the tip and pressing this into the opticalwaveguide path, the inclined surface of the optical waveguide path canbe formed at the identical inclination angle to that of the inclinedsurface of the heat tool and the precision thereof can be raised.

Further, according to the method for producing an optical waveguide ofstill another aspect of the present invention, since the first substrateon which the optical waveguide path is formed is cut at a predeterminedangle and then the end surface formed by the cutting of the firstsubstrate is ground to form the inclined surface of the opticalwaveguide path, the precision of the inclined surface can be raised.

Further, according to the optical waveguide of still another aspect ofthe present invention, since the optical waveguide path is utilized as aspacer interposed between at least one of the light emitting element andlight receiving element and the integrated circuits and the secondsubstrate, at least one of the light emitting element and lightreceiving element and the integrated circuits can be stably secured onthe second substrate.

1. A method of producing an optical waveguide comprising the steps of:forming an optical waveguide path using a first substrate as asupporting base; securing the optical waveguide path supported by thefirst substrate and a second substrate; removing the first substrate soas to transfer the optical waveguide path to the second substrate;forming on said second substrate at least one of a light emittingelement for converting an electrical signal to a light signal and alight receiving element for converting a light signal to an electricalsignal; and forming on said second substrate an integrated circuit fortransferring an electrical signal with said at least one of the lightemitting element and light receiving element, wherein said secondsubstrate comprises an electrical circuit board formed with electricalwiring, and wherein said step of forming an optical waveguide path usingsaid first substrate as a supporting base includes: a step of forming asubstrate separation layer on said first substrate so as to enable thefirst substrate and the optical waveguide path to be separated, and astep of forming an optical waveguide path on said substrate separationlayer, the optical waveguide path being formed to have an inclinedsurface at least at one end of the optical waveguide path.
 2. A methodof producing an optical waveguide as set forth in claim 1, wherein saidstep of forming said optical waveguide path includes: a step of forminga first cladding layer on said substrate separation layer, a step offorming a core layer on said first cladding layer, and a step of forminga second cladding layer on said core layer.
 3. A method of producing anoptical waveguide as set forth in claim 2, wherein the material forforming the first and second cladding layers, has a refractive index oflight smaller than that of the material forming the core layer.
 4. Amethod of producing an optical waveguide as set forth in claim 2,wherein the material forming the first and second cladding layersincludes as a main ingredient at least one material selected from thegroup comprising a polyimide, epoxy resin, acryl resin, polyolefinresin, and synthetic rubber.
 5. A method of producing an opticalwaveguide as set forth in claim 1, wherein the material forming thesubstrate separate layer includes silicon dioxide (SiO₂).
 6. A method ofproducing an optical waveguide as set forth in claim 1, wherein thematerial forming the substrate separation layer includes an etchablemetal material.
 7. A method of producing an optical waveguide as setforth in claim 1, wherein said step of forming an inclined surface atleast at one end of the optical waveguide path includes: a step ofcoating the optical waveguide path with a photoresist and exposing anddeveloping the photoresist so as to form a desired photoresist pattern,a step of inclining an edge region of the photoresist pattern, a step ofusing the photoresist pattern as a mask and anistropically etching theportion of the optical waveguide path exposed from the edge region ofthe photoresist pattern to make the end of the optical waveguide path atapered inclined surface, and a step of removing the photoresistpattern.
 8. A method of producing an optical waveguide as set forth inclaim 1, wherein said step of forming an inclined surface at least atone end of the optical waveguide path includes: a step of vapordepositing on the optical waveguide path a metal film, a step of coatingthe metal film with a photoresist and exposing and developing thephotoresist so as to form a desired photoresist pattern, a step of usingthe photoresist pattern as a mask and etching the meal film to form adesired pattern, a step of using the metal film as a mask andirradiating a laser beam on a predetermined region of the opticalwaveguide path from a predetermined angle to cut the optical waveguidepath, and a step of removing the metal film by etching and cleaning theprocessed product as a whole.
 9. A method of producing an opticalwaveguide as set forth in claim 1, wherein said step of forming aninclined surface at least at one end of the optical waveguide pathincludes: a step of heating a heat tool having an inclined surface atits tip and pressing the tip of the heat tool into the optical waveguidepath so as to form said inclined surface in the optical waveguide pathand a step of removing the heat tool and then polishing off scumproduced at the melted portion of the optical waveguide path.
 10. Amethod of producing an optical waveguide as set forth in claim 1,wherein said step of forming an inclined surface at least at one end ofthe optical waveguide path includes: a step of cutting the firstsubstrate at a predetermined angle and a step of polishing an endsurface formed by the cutting of the first substrate so as to form theend surface of the optical waveguide path into an inclined surface. 11.A method of producing an optical waveguide comprising the steps of:forming an optical waveguide path using a first substrate as asupporting base; securing the optical waveguide path supported by thefirst substrate and a second substrate; removing the first substrate soas to transfer the optical waveguide path to the second substrate;forming on said second substrate at least one of a light emittingelement for converting an electrical signal to a light signal and alight receiving element for converting a light signal to an electricalsignal; and forming on said second substrate an integrated circuit fortransferring an electrical signal with said at least one of the lightemitting element and light receiving element, wherein said secondsubstrate comprises an electrical circuit board formed with electricalwiring, wherein a substrate comprised of a light transmitting materialable to pass light is used as the first substrate, and wherein said stepof securing the second substrate to the optical waveguide path includes:a step of forming at a predetermined position on the second substrate abonding layer comprised of a photo-curing resin which cures byirradiation of light, a step of bringing the bonding layer of the secondsubstrate into close contact with the optical waveguide path formed onthe first substrate, and a step of irradiating light from the back ofthe first substrate toward the second substrate to cure the bondinglayer.
 12. A method of producing an optical waveguide comprising thesteps of: forming an optical waveguide path using a first substrate as asupporting base; securing the optical waveguide path supported by thefirst substrate and a second substrate; removing the first substrate soas to transfer the optical waveguide path to the second substrate;forming on said second substrate at least one of a light emittingelement for converting an electrical signal to a light signal and alight receiving element for converting a light signal to an electricalsignal; and forming on said second substrate an integrated circuit fortransferring an electrical signal with said at least one of the lightemitting element and light receiving element, wherein said secondsubstrate comprises an electrical circuit board formed with electricalwiring, and wherein said step of securing the second substrate to theoptical waveguide path includes: a step of forming at a predeterminedposition on the second substrate a bonding layer comprised of aheat-curing resin, a step of bringing the bonding layer of the secondsubstrate into close contact with the optical waveguide path formed onthe first substrate, and a step of heating the first substrate and thesecond substrate as a whole to cure the bonding layer.
 13. A method ofproducing an optical waveguide as set forth in claim 1, wherein, in thestep of removing the first substrate to transfer the optical waveguidepath to the second substrate, a solvent is supplied between the bondedfirst substrate and second substrate to remove the substrate separationlayer and then the first substrate is separated from the opticalwaveguide path.
 14. A method of producing an optical waveguide as setforth in claim 1, wherein, in the step of removing the first substrateto transfer the optical waveguide path to the second substrate, asolvent is supplied between the bonded first substrate and secondsubstrate to separate the boundary between the substrate separationlayer and the optical waveguide path.
 15. A method of producing anoptical waveguide comprising the steps of: forming an optical waveguidepath using a first substrate as a supporting base; securing the opticalwaveguide path supported by the first substrate and a second substrate;removing the first substrate so as to transfer the optical waveguidepath to the second substrate; forming on said second substrate at leastone of a light emitting element for converting an electrical signal to alight signal and a light receiving element for converting a light signalto an electrical signal; and forming on said second substrate anintegrated circuit for transferring an electrical signal with said atleast one of the light emitting element and light receiving element,wherein said second substrate comprises an electrical circuit boardformed with electrical wiring, wherein spherical portions formed on tipsof fine gold wires are pressed against electrodes of the at least one ofthe light emitting element and light receiving element and saidintegrated circuit, then the spherical portions and the fine gold wiresare pulled apart to cut them and thereby form the connection electrodes,wherein said at least one of the light emitting element and lightreceiving element and said integrated circuit have connectionelectrodes, and wherein, in said step of forming the at least one of thelight emitting element and light receiving element and said step offorming said integrated circuit, at least one of the light emittingelement and light receiving element and the integrated circuit aremounted on the second substrate by flip chip bonding using saidconnection electrodes.